Plane antenna and its designing method

ABSTRACT

A coplanar type circularly polarize wave planar antenna is provided, which can be formed on one side of a dielectric substrate. The planar antenna comprises a dielectric substrate; an almost square radiating element formed on one main surface of the dielectric substrate, the radiating element having notched portions at two corners opposing in one diagonal direction; and a ground conductor formed on the one main surface, the ground conductor having a square opening portion at a center portion thereof and a square outer peripheral shape. The radiating element is placed inside the opening portion of the ground conductor with a gap of a predetermined width being provided with respect to the ground conductor.

TECHNICAL FIELD

The present invention relates generally to a circularly polarized waveantenna for a microwave band used in satellite broadcasts, satellitecommunications and the like, and more particularly to a structure of aplanar antenna suitable to be provided on a window glass of a vehicle.The present invention furthermore relates to a method for designing sucha planar antenna.

BACKGROUND ART

As a circularly polarized wave antenna for a microwave band used insatellite broadcasts, satellite communications and the like, a microstrip antenna (MSA) is prevailing, which is a planar antenna thatincludes a radiating element on the surface of a dielectric substrateand a ground conductor on the back thereof.

In FIG. 1, there is shown one example of the MSA, Reference numeral 10denotes a dielectric substrate, 12 an almost square radiating element,and 14 a ground conductor. Where such MSA is provided on a window glassof a vehicle, the dielectric substrate 10 is structured by the windowglass of a vehicle, the radiating element is patterned on the outersurface of the window glass, and the ground conductor is patterned onthe inner surface of the window glass. Feeder lines are connected to theradiating element and ground conductor, respectively, but the feederline to the radiating element has to be provided passing through thewindow glass, which is hard for the vehicle window glass. Consequently,it is difficult to form the MSA on the window glass of a vehicle.

DISCLOSURE OF THE INVENTION

An object of the present invention is to avoid the problem as describedabove and provide a circularly polarized wave planar antenna of acoplanar type, which may be formed on one side of a dielectricsubstrate.

Another object of the present invention is to provide a method fordesigning the above-described circularly polarized wave planar antenna.

The inventors of the present application have found that, even when theground conductor of the conventional MSA shown in FIG. 1 is moved to thesurface of a glass window on which the radiating element is provided tosurround the radiating element, the structure thus formed functions as acircularly polarized wave antenna.

Hence, a first aspect of the present invention is a planar antennacomprising a dielectric substrate; an almost square radiating elementformed on one main surface of the dielectric substrate, the radiatingelement having notched portions at two corners opposing in one diagonaldirection; and a ground conductor formed on the one main surface, theground conductor having a square opening at a center portion thereof anda square outer peripheral shape; wherein the radiating element is placedinside the opening of the ground conductor with a gap of a predeterminedwidth being provided with respect to the ground conductor.

A second aspect of the present invention is a method for designing theplanar antenna, wherein a diagonal line length in the other diagonaldirection where no notched portions of the radiating element areprovided is deemed as A, the diagonal line length in the one diagonaldirection as B, a width of the gap between the radiating element and theground conductor as G, and a length of one edge of the square peripheralshape of the ground conductor as W, the method comprising the steps ofdeciding the diagonal line length A so that the planar antenna resonateswith a predetermined frequency, deciding the diagonal line length Bbased on a first linear function relationship between a resonancefrequency of the planar antenna and a diagonal line length ratio B/A,deciding said gap width G based on a second linear function relationshipbetween the diagonal line length ratio B/A and a ratio G/A, and decidingthe length W of one edge of the square peripheral shape based on anexponential function relationship between a gradient coefficient of alinear expression representing the second linear function relationshipand a ratio W/A.

In the case where the above-described planar antenna is provided on thewindow glass of a vehicle, the dielectric substrate is a window glass ofa vehicle, and the radiating element and the ground conductor are formedon the inner surface of the window glass.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing one example of a micro strip antenna (MSA);

FIG. 2 is a view showing one embodiment of a planar antenna of thepresent invention;

FIG. 3 is a view showing parameters;

FIG. 4 is a view showing a correlation between a gap width/a diagonalline length ratio G/A and a diagonal line length ratio B/A;

FIG. 5 is a view showing a correlation between a gradient coefficient αand outer one edge length of a ground conductor/a diagonal line lengthW/A; and

FIG. 6 is a view showing a correlation between a resonance frequency anda diagonal line length ratio B/A of a radiating element.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 2 is a view showing one embodiment of a planar antenna of thepresent invention. This planar antenna comprises an antenna pattern asillustrated in the drawing on one main surface of a dielectric substrate10. This antenna pattern is composed of an almost square radiatingelement 16 and a ground conductor 18 which surrounds the radiatingelement and has a square outer peripheral shape. In this case, theradiating element 16 is placed inside a square opening portion 19 formedat the center portion of the ground conductor 18. The radiating element16 has notched portions 20 formed at its two corner portions opposing inone diagonal direction. The reason why such notch portions are formed isto excite a circularly polarized wave as described later. Note that, inthis drawing, reference numeral 22 denotes a feeding point to theradiating element and 24 a feeding point to the ground conductor.Actually, by using a coaxial cable, its core wire is connected to theradiating element, and its braided wire is connected to the groundconductor.

Note that, depending on a relative physical relationship between thefeeding point and the corner notched portion, the radiating direction ofeither a levorotation or a dextrorotation is decided. For example, ifthe physical relationship is constituted as shown in FIG. 2, alevorotation polarized wave is emitted in a direction toward the frontside of the drawing and a dextrorotation polarized wave is emitted in adirection toward the back side of the drawing.

In the planar antenna having such an antenna pattern, importantparameters to decide an antenna performance are a diagonal line lengthratio of the radiating element, a gap width between the radiatingelement and the ground conductor, and the length of one edge of thesquare outer peripheral shape of the ground conductor. In FIG. 3, thereare shown these parameters. The diagonal line length of the portionwhere there are no notched portion of the radiating element is shown byA. Similarly, the diagonal line length of the portion in which there arenotched portions is shown by B. Likewise, the length of one edge of thesquare outer peripheral shape of the ground conductor, that is theground conductor outer one edge length is shown by W, and the gap widthbetween the radiating element and the ground conductor is shown by G. Asdescribed above, by providing notched portions in the radiating element,the diagonal line length ratio varies, and a circularly polarized waveexcitation may be realized.

The present inventors have found by means of simulation that there is acorrelation established among these parameters.

The ratio G/A and the diagonal line length ratio B/A are in a linearrelationship, that is, B/A=α·(G/A)+β, wherein A is one of diagonal linelengths of the relating element as described above and has a correlationwith the resonance frequency f_(R), and G is a gap width as describedabove. The linear relationship is shown in FIG. 4. A coefficient β isherein constant regardless of the ground conductor outer one edge lengthW, while a gradient coefficient α, as shown in FIG. 5, has a correlationwith the ratio W/A of the diagonal line length A to the ground conductorouter one edge length W, thereby establishing an exponential functionrelationship having a correlation with a natural logarithm. Moreover, asshown in FIG. 6, the resonance frequency is in a linear relationshipwith the diagonal line length ratio B/A of the radiating element.

When the correlation among the parameters is used as described above,the design of the planar antenna becomes easy.

Hereinafter, the design procedure will be described with reference tothe flow chart in FIG. 7.

First, the diagonal line length A of the radiating element is decided soas to resonate in the vicinity of a predetermined frequency f_(R) (stepS1).

Next, based on the linear function relationship between the resonancefrequency f_(R) of the planar antenna and the diagonal line length ratioB/A shown in FIG. 6, the diagonal line length B is decided (step S2).

Next, based on the linear function relationship (a linear expression)between the diagonal line length ratio B/A and the ratio G/A of the gapwidth G to the diagonal line length A shown in FIG. 4, the gap width Gis decided (step S3).

Finally, based on the exponential function relationship between thegradient coefficient α of the linear expression used in the step S3 andthe ratio W/A of the ground conductor outer one edge length W to thediagonal line length A shown in FIG. 5, the ground conductor outer oneedge length W is decided.

In this way, the diagonal line length A of the radiating element isdecide so that the radiating element resonates in the vicinity of apredetermined frequency and then each shape parameter is decided so asto establish the above-described correlation, thereby implementing acircularly polarized wave antenna having a good radiatingcharacteristic.

One example of the size of the planar antenna designed as describedabove is shown in Table 1.

An antenna pattern is formed on a glass plate having a thickness of 3.5mm (relative dielectric constant 7). TABLE 1 Resonance frequency G W A B(GHz) (mm) (mm) (mm) (mm) 1.37 0.5 80 42.42 36.17 1.42 1 80 42.42 34.591.46 1.5 80 42.42 33.07

INDUSTRIAL APPLICABILITY

According to the present invention, different from the conventional MSA,all of the antenna patterns may be formed on one side of the dielectricsubstrate and it is, therefore, possible to provide an antenna having agood circularly polarized wave radiating characteristic same as the MSAon a vehicle glass.

Moreover, since the correlation of the shape parameters important fordeciding an antenna performance is clear, the design of the antennabecomes easy.

According to the present invention, therefore, a circularly polarizedplanar antenna of a coplanar type that may be formed on one side of adielectric substrate and a method for designing such a circularlypolarized planar antenna may be realized.

1. A planar antenna, comprising: a dielectric substrate; an almostsquare radiating element formed on one main surface of said dielectricsubstrate, said radiating element having notched portions at two cornersopposing in one diagonal direction; and a ground conductor formed onsaid one main surface, said ground conductor having a square openingportion at a center portion thereof and a square outer peripheral shape;wherein said radiating element is placed inside the opening portion ofsaid ground conductor with a gap of a predetermined width being providedwith respect to said ground conductor.
 2. The planar antenna accordingto claim 1, wherein said dielectric substrate is a window glass of avehicle, and said radiating element and ground conductor are formed onan inner surface of said window glass.
 3. The planar antenna accordingto claim 1, wherein said planar antenna receives a circularly polarizedwave of a microwave band.
 4. A method for designing a planar antenna ofclaim 1, wherein a diagonal line length in the other diagonal directionwhere no notched portions of said radiating element are provided isdeemed as A, the diagonal line length in said one diagonal direction asB, a width of said gap between said radiating element and said groundconductor as G, and a length of one edge of the square peripheral shapeof said ground conductor as W, said method comprising the steps of:deciding said diagonal line length A so that the planar antennaresonates with a predetermined frequency, deciding said diagonal linelength B based on a first linear function relationship between aresonance frequency of the planar antenna and a diagonal line lengthratio B/A, deciding said gap width G based on a second linear functionrelationship between said diagonal line length ratio B/A and a ratio G/Aof said A to said G, and deciding said length W of one edge of thesquare peripheral shape based on an exponential function relationshipbetween a gradient coefficient of a linear expression representing saidsecond linear function relationship and a ratio W/A of said A to said W.5. The method according to claim 4, wherein said dielectric substrate isa window glass of a vehicle and said radiating element and groundconductor are formed on an inner surface of said window glass.
 6. Themethod according to claims 4 or 5, wherein said planar antenna receivesa circularly polarized wave of a microwave band.